The construction of scenarios to investigate alternative future developments
under a set of assumed conditions dates far back.

Scenarios are one of the main tools used to address the complexity and uncertainty
of future challenges. The first scenarios were probably designed to help plan
military operations, often called "war games." Today, scenarios are used regularly
by military organizations around the world for training and planning purposes.
Military strategists and teachers often use very sophisticated computer models
to develop scenarios for a multitude of different purposes.

Scenarios are also increasingly used by enterprises around the world for many
commercial purposes. Perhaps the most famous example is that of the Shell Group
in the wake of the so-called oil crisis, which used scenarios to plan the corporate
response strategies (Jefferson, 1983; Schwartz, 1991). Today, the use of scenarios
is quite widespread.. Many scenarios, particularly those developed for enterprises
in the energy sector, are quantitative and include GHG emissions. Recently,
the World Business Council for Sustainable Development (WBCSD) presented a set
of scenarios developed in collaboration with 35 major corporations (WBCSD, 1998).
The SRES scenario database documents a number of such scenarios that are in
the public domain and have been published.

During the past three decades many global studies have used scenarios as a
tool to assess future CO2 (and in a few cases also other GHG) emissions. One
of the first such global studies was Energy in a Finite World, conducted
by the International Institute of Applied Systems Analysis (IIASA) during the
late 1970s (Häfele et al., 1981). Another influential series of scenarios
that included the assessment of CO2 emissions was developed by the World Energy
Council (WEC, 1993). Recently, IIASA and WEC jointly presented a set of global
and regional scenarios that were developed with a set of integrated assessment
models and then reviewed and revised through 11 regional expert groups (Nakicenovic
et al., 1998b). Another recent set of three scenarios, based on elaborate
narrative stories that described alternative futures, was developed by the Global
Scenario Group (Raskin et al., 1998) and received considerable attention.

Scenarios of future emissions played an important role from the beginning of
the IPCC work. In 1990, the IPCC initiated the development of its first set
of GHG emissions scenarios designed to serve as inputs to general circulation
models (GCMs) and facilitate the assessments of climate-change impacts (Houghton
et al., 1990). Two years later, in 1992, the IPCC approved six new emissions
scenarios (IS92) that provided alternative emissions trajectories for the years
1990 through 2100 for such radiatively active gases as CO2 , carbon monoxide
(CO), methane (CH4), nitrous oxide (N2O), nitrogen oxides (NOx), and sulfur
dioxide (SO2) (Leggett et al., 1992). They were widely used by atmospheric
and climate scientists in the preparation of scenarios of atmospheric composition
and climate change (Alcamo et al., 1995). In many ways, the IS92 scenarios
were pathbreaking. They were the first global scenarios to provide estimates
of the full suite of GHGs and at the time were the only scenarios to provide
emissions trajectories for SO2 . The IS92 scenarios are marked for reference
in many of the illustrations herein that show the variation of emissions and
their driving forces across the scenarios in the SRES database.

An important group of emissions scenarios included in this literature review
was compiled from two international scenario and model comparison activities.
This first group is from the IEW and involves structured comparisons of energy
and emissions scenarios since 1981 (Manne and Schrattenholzer, 1996, 1997).
The participating groups provide information for a standardized scenario poll
from which the ranges and other sample statistics are reported for the main
driving forces and emissions. The other group is the EMF (Weyant, 1993) and
also involves regular scenario comparisons, in addition to standardized input
assumptions, such as the international oil price or carbon emissions taxes.
Both of these international scenario comparison activities provide a large share
of the data for this scenario review and comparison. They include most of the
global and regional emissions scenarios developed by formal modeling approaches.
A large part of these activities is based on the use of scenarios for the purpose
of climate-change research. A third scientific effort that involves scenario
comparisons is the Energy Technology Systems Analysis Programme (ETSAP; Kram,
1993) supported by the International Energy Agency (IEA). The ETSAP work involves
scenario analysis by more than 40 scientific groups from about 20 countries
using the same modeling approach.

In addition to the many scientific, governmental, and private organizations
throughout the world engaged in scenario-building, some international governmental
organizations regularly develop global and regional scenarios that include GHG
emissions. For example, the IEA regularly publishes global energy scenarios
that include CO2 emissions (IEA, 1998). Most of these scenarios are of shorter
term and so are not suitable for the requirements of IPCC (see Chapter
1 for further details). Nevertheless, they are included in this assessment
to facilitate a more comprehensive evaluation of emissions and their driving
forces during the next few decades.

Some studies consider scenarios that involve explicit policies and measures
to reduce emissions of GHGs or adapt to climate change. Such climate change
intervention, control, or mitigation scenarios are an important tool for the
assessment of policies and measures that would be required to reduce future
GHG emissions. In this report, we use the terminology from the most recent IPCC
evaluation of emissions scenarios (Alcamo et al., 1995). Those scenarios
that include some form of policy intervention are referred to as intervention
scenarios, while those that do not assume any climate policy measures, such
as the 40 SRES scenarios, are referred to as non-intervention scenarios.
In some cases, intervention scenarios go even further and investigate more radical
emissions reductions required to stabilize atmospheric concentrations of these
gases (in accordance with Article 2 of the United Nations Framework Convention
on Climate Change (UNFCCC, 1992)). In contrast, the SRES scenarios do not include
any explicit additional climate policy initiatives in accordance with the Terms
of Reference (see Appendix I).

The SRES writing team used a general approach to identify intervention scenarios.
According to this approach, a scenario is identified as an intervention scenario
if it meets one of the following two conditions:

it includes explicit or implicit policies and/or measures of which the primary
goal is to reduce GHG emissions (e.g., a carbon tax or a policy encouraging
the use of renewable energy).

Note that this classification system is only a first step, and further work
is needed to refine this taxonomy.

Some scenarios in the literature are difficult to classify as intervention
or non-intervention, such as those developed to assess sustainable development.
These studies consider futures that require radical policy and behavioral changes
to achieve a transition to a sustainable development path; Greenpeace formulated
one of the first (Lazarus et al., 1993). This class of scenarios describes
low emissions futures that sometimes, but not always, result from specific climate
policy measures. Such sustainable development scenarios are also included in
this assessment of the scenario literature. Where they do not include the explicit
policies of the SRES criteria, they can be classified as non-intervention scenarios.
However, there is a great deal of ambiguity as to what constitutes policies
directed at climate change, as opposed to those directed at achieving sustainable
development in general. Thus, some of these sustainable development scenarios
are "non-classified" (i.e., the information available is insufficient to determine
whether or not the scenarios included any additional climate policy initiatives).